linux-kernel - Re: [PATCH v7 2/8] x86/crash: Introduce new options to support cpu and memory hotplug

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Date:   Tue, 26 Apr 2022 09:51:25 +0530
From:   Sourabh Jain <sourabhjain@...ux.ibm.com>
To:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v7 2/8] x86/crash: Introduce new options to support cpu
 and memory hotplug


On 13/04/22 22:12, Eric DeVolder wrote:
> CRASH_HOTPLUG is to enable cpu and memory hotplug support of crash.
>
> CRASH_HOTPLUG_ELFCOREHDR_SZ is used to specify the maximum size of
> the elfcorehdr buffer/segment.
>
> This is a preparation for later usage.
>
> Signed-off-by: Eric DeVolder <eric.devolder@...cle.com>
> Acked-by: Baoquan He <bhe@...hat.com>
> ---
>   arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index b0142e01002e..f7b92ee1bcc7 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2072,6 +2072,32 @@ config CRASH_DUMP
>   	  (CONFIG_RELOCATABLE=y).
>   	  For more details see Documentation/admin-guide/kdump/kdump.rst
>   
> +config CRASH_HOTPLUG
> +	bool "kernel updates of crash elfcorehdr"
> +	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG) && KEXEC_FILE
> +	help
> +	  Enable the kernel to update the crash elfcorehdr (which contains
> +	  the list of CPUs and memory regions) directly when hot plug/unplug
> +	  of CPUs or memory. Otherwise userspace must monitor these hot
> +	  plug/unplug change notifications via udev in order to
> +	  unload-then-reload the crash kernel so that the list of CPUs and
> +	  memory regions is kept up-to-date. Note that the udev CPU and
> +	  memory change notifications still occur (however, userspace is not
> +	  required to monitor for crash dump purposes).
> +
> +config CRASH_HOTPLUG_ELFCOREHDR_SZ
> +	depends on CRASH_HOTPLUG
> +	int
> +	default 131072
> +	help
> +	  Specify the maximum size of the elfcorehdr buffer/segment.
> +	  The 128KiB default is sized so that it can accommodate 2048
> +	  Elf64_Phdr, where each Phdr represents either a CPU or a
> +	  region of memory.
> +	  For example, this size can accommodate a machine with up to 1024
> +	  CPUs and up to 1024 memory regions, eg. as represented by the
> +	  'System RAM' entries in /proc/iomem.

Is it possible to get rid of CRASH_HOTPLUG_ELFCOREHDR_SZ?

How about finding the additional buffer space needed for future CPU and memory
add during the kdump load? Not sure about the feasibility of doing this in
kexec tool (userspace).

Thanks,
Sourabh Jain